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<article class='withtoc'>
    <h1>
      直接使用Image Pipeline
      <a class="edit-page-link" href="https://github.com/liaohuqiu/fresco-docs-cn/blob/docs/docs/02-using-image-pipeline.md" target="_blank">编辑和纠错</a>
    </h1>
    <p></p>

    <p>本页介绍Image pipeline的高级用法，大部分的应用使用<a href="using-drawees-xml.html">Drawees</a> 和image pipeline打交道就好了。</p>

<p>直接使用Image pipeline是较为有挑战的事情，这意味着要维护图片的内存使用。Drawees
会根据各种情况确定图片是否需要在内存缓存中，在需要时加载，在不需要时移除。直接使用的话，你需要自己完成这些逻辑。</p>

<p>Image pipeline返回的是一个<a href="closeable-references.html">CloseableReference</a>对象。在这些对象不需要时，Drawees会调用<code>.close()</code>方法。如果你的应用不使用Drawees，那你需要自己完成这个事情。</p>

<p>Java的GC机制会在Bitmap不使用时，清理掉Bitmap。但要GC时总是太迟了，另外GC是很昂贵的开销。GC大对象也会带来性能问题，尤其是在5.0以下系统。</p>

<h4>调用 pipeline</h4>

<p>首先<a href="image-requests.html">创建一个image request</a>. 然后传递给 <code>ImagePipeline:</code></p>
<div class="highlight"><pre><code class="language-java" data-lang="java"><span class="n">ImagePipeline</span> <span class="n">imagePipeline</span> <span class="o">=</span> <span class="n">Fresco</span><span class="o">.</span><span class="na">getImagePipeline</span><span class="o">();</span>
<span class="n">DataSource</span><span class="o">&lt;</span><span class="n">CloseableReference</span><span class="o">&lt;</span><span class="n">CloseableImage</span><span class="o">&gt;&gt;</span> 
    <span class="n">dataSource</span> <span class="o">=</span> <span class="n">imagePipeline</span><span class="o">.</span><span class="na">fetchDecodedImage</span><span class="o">(</span><span class="n">imageRequest</span><span class="o">);</span>
</code></pre></div>
<p>关于如果接收数据，请参考<a href="datasources-datasubscribers.html">数据源</a> 章节。</p>

<h4>忽略解码</h4>

<p>如果你不保持图片原始格式，不执行解码，使用<code>fetchEncodedImage</code>即可:</p>
<div class="highlight"><pre><code class="language-java" data-lang="java"><span class="n">DataSource</span><span class="o">&lt;</span><span class="n">CloseableReference</span><span class="o">&lt;</span><span class="n">PooledByteBuffer</span><span class="o">&gt;&gt;</span> 
    <span class="n">dataSource</span> <span class="o">=</span> <span class="n">imagePipeline</span><span class="o">.</span><span class="na">fetchEncodedImage</span><span class="o">(</span><span class="n">imageRequest</span><span class="o">);</span>
</code></pre></div>
<h4>从Bitmap缓存中立刻取到结果</h4>

<p>不像其他缓存，如果图片在内存缓存中有的话，可以在UI线程立刻拿到结果。</p>
<div class="highlight"><pre><code class="language-java" data-lang="java"><span class="n">DataSource</span><span class="o">&lt;</span><span class="n">CloseableReference</span><span class="o">&lt;</span><span class="n">CloseableImage</span><span class="o">&gt;&gt;</span> <span class="n">dataSource</span> <span class="o">=</span>
    <span class="n">mImagePipeline</span><span class="o">.</span><span class="na">fetchImageFromBitmapCache</span><span class="o">(</span><span class="n">imageRequest</span><span class="o">);</span>
<span class="n">CloseableReference</span><span class="o">&lt;</span><span class="n">CloseableImage</span><span class="o">&gt;</span> <span class="n">imageReference</span><span class="o">;</span>
<span class="k">try</span> <span class="o">{</span>
  <span class="n">imageReference</span> <span class="o">=</span> <span class="n">dataSource</span><span class="o">.</span><span class="na">getResult</span><span class="o">();</span>
  <span class="k">if</span> <span class="o">(</span><span class="n">imageReference</span> <span class="o">!=</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
    <span class="n">CloseableImage</span> <span class="n">image</span> <span class="o">=</span> <span class="n">imageReference</span><span class="o">.</span><span class="na">get</span><span class="o">();</span>
    <span class="c1">// do something with the image</span>
  <span class="o">}</span>
<span class="o">}</span> <span class="k">finally</span> <span class="o">{</span>
  <span class="n">dataSource</span><span class="o">.</span><span class="na">close</span><span class="o">();</span>
  <span class="n">CloseableReference</span><span class="o">.</span><span class="na">closeSafely</span><span class="o">(</span><span class="n">imageReference</span><span class="o">);</span>
<span class="o">}</span>
</code></pre></div>
<p>千万 <strong>不要</strong> 省略掉 <code>finally</code> 中的代码!</p>

<h4>预加载图片</h4>

<p>预加载图片可减少用户等待的时间，如果预加载的图片用户没有真正呈现给用户，那么就浪费了用户的流量，电量，内存等资源了。大多数应用，并不需要预加载。</p>

<p>Image pipeline 提供两种预加载方式。</p>

<p>预加载到文件缓存:</p>
<div class="highlight"><pre><code class="language-java" data-lang="java"><span class="n">imagePipeline</span><span class="o">.</span><span class="na">prefetchToDiskCache</span><span class="o">(</span><span class="n">imageRequest</span><span class="o">);</span>
</code></pre></div>
<p>预加载到内存缓存:</p>
<div class="highlight"><pre><code class="language-java" data-lang="java"><span class="n">imagePipeline</span><span class="o">.</span><span class="na">prefetchToBitmapCache</span><span class="o">(</span><span class="n">imageRequest</span><span class="o">);</span>
</code></pre></div>

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